The present invention relates to the manufacture of sputtering targets, more particularly to the manufacture of sputtering targets from volatile and thermally unstable metal oxides, and even more particularly to the manufacture of indium oxide and tin oxide sputtering targets.
Thin films are often produced using plasma sputtering techniques. Targets are used during the sputtering process as the source of material for the film being deposited onto a substrate. Targets made from volatile and thermally unstable metal oxides have been used to sputter thin films which exhibit properties useful in a variety of applications. For example, thin films of electro-optic materials, such as indium oxide and tin oxide, are known to exhibit high transmittance and low resistivity. These materials are commonly used as electro-conductive films in electroluminescence (EL) displays, liquid crystal displays, solar cells, defrost/defog heaters for airplanes and the like.
Known methods of making sputtering targets from such volatile and thermally unstable metal oxides have included consolidating, such as by hot-pressing, powders of the metal oxide target starting material, like In2O3 and SnO2, into a target blank in an inert gas environment. Graphite or ceramic die assemblies with a die cavity typically have been used to press the powdered material. In a number of these prior processes, in particular prior processes for making indium oxide and tin oxide (ITO) targets, the metal oxide powder was loaded into the die cavity such that the powder was in direct contact with the walls of the die cavity. After consolidation, the resulting target blank was typically formed, such as by machining, grinding, polishing, etc., into a finished target. Detailed descriptions of a number of prior processes for making ITO sputtering targets may be found in U.S. Pat. Nos. 5,160,675 and 5,094,787 and in Japanese Patent Nos. 04341504, 04293769, 04154654, 04074860, 03199373 and 02043356.
The production of metal oxide targets, notably ITO targets, using graphite die assemblies have been known to exhibit a number of chronic problems associated with interaction between the powdered target starting material and the graphite material of the die assembly. The problems have included full reduction of the metal oxide to metal, at least in the form of a layer on the outer surface of the resulting target blank. Removal of this metal layer is generally necessary before the target is suitable for use. However, removal of the reduced metal, for example, a layer of indium or indium-tin alloy on an ITO target, often results in cracks in the target or fracturing of the target, which then must be scrapped. Also, the formation of cracks in the target reduces its resistance to fracturing during use. Lower pressing temperatures and/or shorter pressing times have been used in order to avoid the formation of a reduced metal layer while still using a graphite die assembly to press the powder. Pressing at lower temperatures and/or for shorter times typically results in poor consolidation of the powdered target starting material, which in turn results in targets with low density, low strength and/or a low resistance to fracturing (i.e., low toughness).
The density, strength and toughness of a target are often very important to the target""s performance. Reportedly, high density ITO targets are required in order to sputter high quality ITO thin films, in particular, thin films free from particles. In addition, good strength and toughness are typically necessary to successfully form the ITO target blank into the desired final target shape. ITO targets with good toughness are also less likely to fracture during the sputtering process. Such fracturing can result in relatively large particles being deposited onto the substrate, often generating defects in the ITO film. Such defects in the ITO film can adversely affect the electro-optic properties of the film.
ITO and other such metal oxide sputtering targets produced with die assemblies made of a ceramic, such as Al2O3 or ZrO2, are less likely to exhibit the problems noted above when graphite die assemblies are used. However, such ceramic die assemblies are generally more expensive to manufacture, less resistant to thermal shock and not as thermally conductive as comparable graphite die assemblies. Ceramic dies are thus more likely to crack during use, require longer heating and soaking times during the hot-pressing operation, and generally increase the costs of the target manufacturing process.
Therefore, there is a need for a more cost effective method and apparatus for manufacturing denser, stronger and tougher sputtering targets made from volatile and thermally unstable metal oxides.
The present invention is directed to a less expensive apparatus and process for consistently making acceptable sputtering targets by hot-pressing metal oxides that are volatile and thermally unstable at the hot-pressing temperature.
A general aspect of the present invention is directed to such an apparatus and process using a graphite die assembly.
A particular aspect of the present invention is directed to such an apparatus and process for consistently making relatively higher density, higher strength and tougher indium oxide and tin oxide (ITO) sputtering targets using a graphite die assembly.
According to the general principles of the present invention, a powder of a first material, also termed a target starting material, which includes metal oxide particles, is surrounded by a barrier material, as the first material is hot-pressed at an elevated temperature into a target blank in an oxygen-free, preferably inert, gas environment. The metal oxide particles in the powdered target starting material are volatile and thermally unstable at the elevated hot-pressing temperature. The barrier material is not in powder form, but is in the form of a solid sleeve which is thermally stable at elevated temperatures. In the present invention, first the barrier sleeve is placed in the die cavity of a graphite die assembly, followed by the target starting material. While in the die cavity encased by the barrier sleeve, the target starting material is bonded together (or sintered) by applying a sufficiently high temperature and pressure, for a long enough period of time, to produce a metal oxide sputtering target having a density, strength and toughness suitable for sputtering films of desirable quality. The barrier material sleeve is intended to substantially prevent a reducing gas from penetrating therethrough and reaching the unstable target starting material during the hot-pressing operation, thereby preventing any significant degree of full reduction of the metal oxide target material to metal. Preferably, the barrier material sleeve is also sufficient to substantially protect the graphite die assembly from attack and degradation by any dissociated components of the metal oxides during hot-pressing.
At elevated hot-pressing temperatures, the volatile and thermally unstable metal oxides break down or dissociate into oxygen gas and reduced oxides which may or may not be volatile, depending upon the temperature attained. For example, when the ITO material is subjected to hot-pressing temperatures of greater than or equal to about 850xc2x0 C., the In2O3 may begin to dissociate into free and reactive oxygen gas and other lower order indium oxides. Depending upon the hot-pressing temperature, some of these other indium oxides, such as In2O, may vaporize, while the balance of the indium oxide remains stable (i.e., in solid form). The dissociation of In2O3 is evidenced by color changes in the ITO material. In2O3 is yellow, while In2O is black. Typically, it is desirable for the target to have a uniform color which is indicative of a uniform composition. For at least some ITO targets, a uniform dark blue color is preferred. The present invention enables targets with such a uniform color to be consistently produced in a cost effective manner.
In the absence of the protection afforded by the barrier material sleeve, hot-pressing at such elevated temperatures will likely result in the oxygen (O) gas reacting with carbon (C) from the graphite die assembly, generating carbon monoxide (CO) gas. The CO gas will in turn react with In2O3 and other indium oxides, thereby forming metallic indium (In) and carbon dioxide (CO2). The resulting indium metal melts and tends to diffuse into the remaining oxide material such that the target blank has an outer coating of this reduced metal with an inner core of any remaining metal oxide. Because the indium metal is difficult to remove without cracking or fracturing the target blank, such blanks are typically unsuitable for further processing into finished targets and are scrapped. In addition, reduction of the ITO material has been found to shorten the life of the graphite die due to oxidation of the graphite by the liberated oxygen gas. Oxidation of the graphite may cause the die to crack during hot-pressing. Thus, it has been found that by isolating the powdered ITO target starting material with an appropriate barrier material sleeve, reduction of the indium oxide to indium metal during hot-pressing and the problems associated therewith may be eliminated or at least significantly reduced.
In one embodiment of the ITO target manufacturing apparatus and process of the present invention, a barrier material sleeve surrounds or encases the powdered ITO target starting material in order to better ensure that an adequate barrier is provided. The barrier sleeve may be a metal oxide, such as Al2O3, ZrO2, TiO2, MgO and combinations thereof, or a non-oxide ceramic, such as SiC, SiN, and combinations thereof.
Prior attempts at avoiding significant reduction of the powdered ITO target starting material, which would enable continued use of graphite die assemblies, have included lowering the hot-pressing temperature and/or shortening the time duration of the hot-pressing operation. However, such process changes typically resulted in target blanks having undesirable mechanical properties (i.e., low density, strength and toughness). Implementation of the barrier sleeve according to the present invention has enabled much higher pressing temperatures and longer pressing times to be used with graphite dies while significantly decreasing the amount of ITO material reduced to metal, as well as decreasing oxidation of the graphite dies. In this way, relatively low cost targets having improved physical properties may be manufactured.
Even when ITO and other metal oxide target starting materials are protected from reducing gases as described above, the starting material still may be partially reduced, thereby dissociating vaporous and gaseous components. The extent of such dissociation typically depends on the pressing temperature and the time at that temperature. It has been determined that when these vapors and gases are trapped inside the powdered target material and not allowed to escape, lower quality targets may be produced. Trapped vapor or gas tends to cause inconsistent consolidation of the powdered target material (i.e., bonding between powder particles), thereby lowering the density, strength and toughness of the final target. Such retained vapor or gas also tends to cause poor color uniformity in the resulting target. It is believed that such discoloration may be due, at least in part, to the trapped vapors and gases recombining into higher order oxides of different color(s) when the target blank is cooling.
It has been found that smaller targets do not exhibit the above noted problems associated with such retained gases to the same extent as larger targets. It is believed that there are two primary causes for this difference. For larger targets, more powdered target starting material is used which causes a proportional increase in the production of dissociated vaporous and gaseous components. In addition, because of the target""s larger size, the vapors and gases produced near the center of the powdered target material have further to travel to exit from the die cavity. This is best understood with reference to a typical graphite die assembly, which includes a cylindrical die tube or ring and two opposing cylindrical die punches that slide longitudinally within the die ring, compressing the powder therebetween. The interior of the die cavity is defined by the leading surfaces of the two die punches and the inside surface of the die ring, with the only escape for vapor or gas being between the die punches and the die ring. Generally, this is insufficient to vent trapped vapors and gases in larger targets.
To avoid the drawbacks of retaining vapors and gases in the powdered target material during hot-pressing, an optional feature of the present apparatus and method was developed to minimize the amount of trapped vapor and gas. This feature includes a gas release device which facilitates the venting of vapor and gas from the die cavity. In general, the gas release device has two surfaces connected by a plurality of pathways. The gas release device is disposed within the die cavity such that one surface faces the powdered target material and the other surface faces at least one wall of the die cavity, or a barrier sleeve within the die cavity. The pathways in the gas release device are constructed so that vapor and gas generated during hot-pressing flows from the powdered target material, through the pathways and out of the die cavity between the die punches and the die ring.
In one embodiment, the gas release device is a perforated disk having two flat surfaces. Two such disks are positioned in the die cavity, each with one surface facing a respective die punch, or a barrier sleeve element within the die cavity, and the other surface facing the powdered target material. Each disk has a plurality of channels or grooves formed in the surface facing the die punch or barrier sleeve element. Each groove leads to the outer edge or periphery of the disk. A plurality of through holes connect these grooves to the surface of the disk facing inwardly toward the target material. In this way, vapor and gas escaping from the powdered target material can flow through the holes, along the grooves to the outer periphery of the disk, and out of the die assembly between the die punches and the die ring or barrier sleeve. By venting more of the dissociated vapors and gases, targets may be produced which exhibit higher densities, greater strength, and better toughness, while maintaining more uniform color.
Yet another feature of the present invention further reduces the amount of vapor and gas retained in the powdered target material during hot-pressing. This is accomplished by subjecting the target starting material to a heat treatment before the hot-pressing operation. This heat treatment involves heating the starting powder to a temperature which causes partial reduction of the metal oxide without bonding the powder particles together (i.e., sintering) to any significant degree. In this way, at least some of the dissociated vaporous and gaseous components of the target starting material are driven off before the hot-pressing operation. Thus, smaller amounts of vapor and gas are produced during hot-pressing and captured within the powdered target starting material. This preliminary heat treatment should be performed in an oxygen-free environment, such as an inert gas atmosphere. Preferably, a mild reducing environment can be used to accelerate this partial reduction while avoiding any significant grain growth (i.e., sintering) or formation of reduced metal. For example, a sheet of graphite foil may be introduced into the heat treatment environment to cause this acceleration. Thus, by partially reducing the powdered target material before hot-pressing, targets having even higher densities, greater strength, better toughness and more uniform color may be obtained.
In the present target manufacturing apparatus and process, the barrier material sleeve is intended to provide a barrier for the powdered metal oxide target material and substantially prevent contact with any reducing gas coming from the graphite die material. This barrier sleeve eliminates or at least significantly avoids full reduction of the metal oxide into metal, as well as oxidation of the graphite die material. Thus, the present invention makes it possible to use graphite die assemblies to hot-press powders of ITO materials, and similar volatile and thermally unstable metal oxides, into denser, stronger and tougher sputtering targets than heretofore thought possible. In addition, a gas release feature of the present invention significantly reduces the amount of vapor or gas trapped in the powdered target starting material during hot-pressing, thus enabling such targets to be made with even better properties, including more uniform color. To further reduce the amount of these vapors and gases trapped in the powdered target starting material, and thereby further improve the target properties, the powdered target starting material may be subjected to a heat treatment to drive off a portion of such volatiles before hot-pressing. Utilizing the principles of the present invention, ITO sputtering targets have been produced which exhibit a minimum density of about 90% (6.45 g/cc) of theoretical density and a minimum flexure strength of about 90 MPa, with high crack resistance (i.e., toughness) and color uniformity.